I think the Geiger xenos take this (with even numbers).
1. The Geigers are on home turf and prepared (they're better at hiding - stalking - striking).
2. Any PBer bites or stabs a Geiger gets acid-burned very badly (and I see no reason to assume the acid blood wouldn't hurt them).
3. The Geigers seem smarter. Heck if they figure out what Riddick did (swaying to stay unseen), then even with superior numbers, the PBers are toast.